[ThinLTO] Make llvm-lto module ID numbering consistent with linkers

The module ID numbering typically starts at 0 (in both the new and old
LTO APIs, used by linkers). Make llvm-lto consistent with that.

Split out of D46699.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@332476 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed